The present invention relates to an illuminating optical unit in an image display unit for displaying an image by use of light modulated by a spatial modulation device, and to the image display unit. More particularly, the invention relates to a technology for enhancing the quality of an image.
As a form of image display unit, hitherto there has been proposed a projection display in which a screen is irradiated with a light beam modulated by a spatial modulation device, such as a liquid crystal panel, a DMD (Digital Micromirror Device), etc., to display an image on the screen. In such a projection display, conventionally, a lamp, such as a metal halide lamp, a halogen lamp and a xenon lamp, has been used as a light source.
Where such a lamp is used as a light source, however, there is the problem that the life of the light source is short and, hence, maintenance thereof is troublesome. In addition, an optical system for picking up three primary colors of light from the white light generated from the lamp is complicated, color reproduction region is limited and light utilization efficiency is lowered.
In order to solve the above problems, it has been attempted to use a laser light source, such as a semiconductor laser, as a light source for a projection display. A laser light source has a longer life as compared with lamps, and has an excellent directionality, which enables efficient utilization of the light radiated, thus promising a high energy utilization efficiency. In addition, the laser light source has a number of merits such as its monochromatic property, which permits a wide color reproduction region.
Besides, in recent years, attention has been paid to the development of a display using an active drive type grating (diffraction grating) based on the micro-machine technology. The grating type device used here is called a “Grating Light Valve” (hereinafter abridged to “GLV”), and has the advantages that, as contrasted to the case of using a conventional spatial modulator, it is possible to display a seamless, sharp and bright image, the device can be manufactured inexpensively by use of the micro-machine technology, the device can perform a high-speed operation, and so on.
It has been contemplated to use a laser light source, such as the above-mentioned semiconductor laser, as a light source for illuminating the above-mentioned GLV.
However, where a laser light source is used as a light source for an image display unit such as the above-mentioned projection display, there occurs the problem that speckles (or speckle noise) are generated, which degraded the image quality.
The speckle noise is a phenomenon generated through the process in which a coherent beam with an aligned phase from a laser light source is scattered by a random phase plane (object surface) and disturbed wave fronts coming from adjacent regions of the object surface interfere with each other on an observation plane; thus, the speckle noise appears on the observation plane as a granular intensity distribution.
In the projection display using the laser light source, when such a speckle noise as above is generated between the screen constituting the object surface and the observer's eye (retina) constituting the observation plane, the observer recognizes it as a degradation of an image. Therefore, how to suppress the speckle noise is important in realizing an image display unit such as a projection display using a laser light source.
Incidentally, there is a laser beam with a low spatial coherence, and it may be considered that the use of such a laser beam for an image display unit such as a projection display can realize a two-dimensional image with little speckle noise. However, the laser beam with a low spatial coherence lacks stability, i.e., it is liable to generate noise and the beam shape and intensity distribution thereof are liable to vary. Thus, the laser beam with a low spatial coherence is unsuitable for use as a light source for the image display unit such as a projection display.
In the technological status at present, a visible laser beam which is stable in beam shape, intensity distribution and the like and which has a high output is generally high in spatial coherence, leading to the above-mentioned problem of speckle noise.